Field of the Invention
This invention relates to lighting and display fields, and in particular, it relates to a light emitting device, light emitting device assembly and related projection system.
Description of the Related Art
Conventional phosphor wheels include transmission type and reflection type color wheels. The reflective type phosphor wheel is formed by attaching phosphor materials on a reflective substrate; the excitation light illuminated on the phosphor material is converted into a converted light, which is reflected by the reflective substrate. Thus, the direction of the output converted light of the phosphor wheel is the opposite of the direction of the excitation light illuminating onto the phosphor wheel. In other words, the output side and the input side are the same side of the phosphor wheel. The transmission type phosphor wheel is formed by attaching phosphor materials on a transparent substrate; the excitation light illuminated on the phosphor material is converted into a converted light, which transmits through the transparent substrate. Typically, a filter that transmits the excitation light and reflects the converted light is provided on the input side of the phosphor wheel. Thus, the direction of the output converted light of the phosphor wheel is the same as the direction of the excitation light illuminating onto the phosphor wheel. In other words, the output side and the input side are two different sides of the phosphor wheel.
Referring to FIGS. 1a and 1b; FIG. 1a is a side view of a conventional phosphor wheel, and FIG. 1b is top plan view of the wavelength conversion device of the phosphor wheel of FIG. 1a. As shown in FIGS. 1a and 1b, the phosphor wheel includes a wavelength conversion device 10 containing phosphor materials and a motor 20. The wavelength conversion device 10 includes a substrate 11 and a wavelength conversion layer 12 stacked together, and the substrate is directly in contact with the motor 20. The surface of the motor 20 that directly contacts the substrate is the drive surface of the motor. The substrate has a circular plate shape, and the motor 20 has a cylindrical shape coaxial with the circular plate, for driving the wavelength conversion device 10 to rotate around the center of the circular plate. The wavelength conversion layer 12 has a ring shape coaxial with the substrate; at least one type of phosphor material is provided on the ring shape for receiving the excitation light A and converting it to converted light B.
By studying the conventional phosphor wheels, the inventors of this invention discovered that, if the phosphor wheel is the transmission type, to allow the excitation light to pass through the wavelength conversion layer without being blocked by the motor, the inner radius of the wavelength conversion layer 12 needs to be larger than the radius of the drive surface of the motor, so that the area of the wavelength conversion device is larger than the sum of the area of the motor's drive surface and the area of the wavelength conversion layer 12. Similarly, when a reflective type phosphor wheel is used, and the excitation light illuminates on the wavelength conversion device from a side of the wavelength conversion device facing the motor, then the area of the wavelength conversion device 10 needs to be larger than the sum of the area of the motor's drive surface and the area of the wavelength conversion layer 12. It can be seen that the area of the wavelength conversion device 10 needs to be larger than the sum of the area of the motor's drive surface and the area of the wavelength conversion layer 12, so it is difficult to use such phosphor wheels in products that require a relatively small thickness.